As an independent wheel suspension system for an automobile, there is known the double wishbone suspension system in which a hub carrier rotatably supporting a wheel is coupled to the vehicle body by way of a plurality of lateral links which are arranged substantially in the manner of a parallel link mechanism with one link formed by the hub carrier and the opposite link formed by the vehicle body. In some cases, one of the lateral links is formed substantially as a trailing arm. Also is known the type of suspension system using a trailing arm and three lateral links, such as the one disclosed in Japanese patent laid open publication No. 60-53408. A suspension system of these types not only permit a favorable control of the changes in the toe displacement and the camber displacement of a wheel as the wheel undergoes a vertical displacement but also provides a positive supporting of the wheel against the longitudinal load as well as a favorable compliance control.
In these wheel suspension systems, a shock absorber which typically combines a coil spring and a hydraulic damper arranged in a coaxial or parallel relationship is used for reducing the impacts which are applied to the wheel. This shock absorber is arranged vertically in relation with the vehicle body, and, typically, is attached to the vehicle body at its upper end and to an intermediate part of one of the lateral links at its lower end.
However, a shock absorber is relative long in length so as to accommodate the full stroke of the vertical motion of the wheel, and this dictates the vertical dimension of the wheel suspension system. For instance, German Patent No. 2137757 discloses a lateral or, in other words, horizontal arrangement of a shock absorber consisting of a coil spring and a hydraulic damper along the widthwise or lateral direction of the vehicle to the end of improving the dynamic properties of the suspension system by mutually interconnecting a part of the linkage systems of the right and the left suspension system. This lateral arrangement of a shock absorber permits reduction of the height of the suspension system depending on the overall arrangement but the invention of this German patent publication is intended to improve the performance of the suspension system by interconnecting the two suspension systems with a highly complex linkage system without any particular reference to the possibility of reducing the height of the suspension system.
Normally, a shock absorber is accommodated in a wheel house in a vertical orientation and is thereby shielded from the impacts of external objects, but a laterally arranged shock absorber exposes the side surfaces of its damper and coil spring under the bottom floor panel of the vehicle body. Therefore, particularly in a front wheel drive vehicle, small stones which fly up from the front wheels could damage them. To prevent this problem, it is conceivable to provide an under-guard on the undersurface of the vehicle body, but it leads to an increase in the manufacturing cost and the weight of the vehicle.
Moreover, conventional wheel suspension systems using vertically arranged shock absorbers have the disadvantage that the lateral link connected to the lower end of the shock absorber is required not only to support side forces but also the weight of the vehicle, and is accordingly required to be sufficiently massive to ensure the necessary mechanical strength.
Further, such a shock absorber is typically attached to a vehicle body by way of an elastic bush such as a rubber bush for reducing the transmission of vibrations and noises to the vehicle body. Typically, one end of the damper is also pivotally coupled to the vehicle body so as to permit the change in the angular position of the damper resulting from the vertical motion of the wheel. However, in pivotally supporting such a shock absorber, since the reaction from the weight of the vehicle acting upon the coil spring and the hydraulic damper is applied to the pivoted part, the compliance of the elastic bush which is interposed in the pivoted portion is required to be low in order to ensure a sufficient load bearing capability. This diminishes the capability of the elastic bush to isolate the vehicle body from vibrations and could impair the riding comfort of the vehicle.
Conventionally, the direction of the extending and compressing motion of the shock absorber coincided with the direction of the vertical motion of the wheel, and the load acting upon the wheel generally changed linearly with the displacement of the shock absorber. However, it is desirable to achieve a progressive property in which the reaction from the suspension system progressively increases as the wheel moves towards a full bump condition without unduly complicating the suspension structure. This permits reduction of the effective spring constant near the neutral position of the shock absorber without making the shock absorber any more prone to bottom out.
Further, as means for elastically defining a limit of the stroke of the damper, it is known to interpose a cushion member consisting of an elastic member normally called as a bump stop rubber between the outer tube end and the piston rod end of the damper so as to prevent the abrupt occurrence of a fully compressed state or a bottomed state of the damper. When the wheel moves into a full bump condition, the bump stop rubber intervenes by placing a limit to the bump stroke of the suspension system, and the spring reaction changes so suddenly that the occurrence of a severe tossing shock cannot be avoided. When this bump stop rubber has become effective, it produces the same effect as increasing the spring constant of the coil spring. Such a tossing shock should be reduced.
If a trailing arm is used in such a suspension structure, since the hub carrier is supported by a trailing arm which extends from a front part of the vehicle body, a relatively large torque acts upon the hub carrier which tends to turn the hub carrier in the direction to vary the toe angle as a result of the lateral and longitudinal forces which are applied to the wheel at its area of contact with the road surface. Therefore, it becomes necessary to increase the mechanical strength of the lateral links and to reduce the compliance of the rubber bushes, and a desirable toe control becomes difficult.